                1. For batteries to be used for applications such as vehicle starter, electric bikes, electric motorcycles, electric or hybrid vehicles, etc, high voltage is essential owing to the increase of efficiency and the decrease of cost. The increase of voltage requires batteries to be connected in series.        2. Problems associated with batteries in series are:                    a. when one battery has a lower capacity, the capacity of the overall set of batteries is dictated by the capacity of the battery of lower capacity;            b. if the battery possessing the lower capacity can not be charged to full capacity during charging, the performance of the entire battery set will be degraded owing to the lower capacity battery. This is known in the art as cell imbalance;            c. the lower capacity of one specific battery can be caused by either high self discharge or defects during battery production.                        3. Conventional ways to solve the cell imbalance problem are:                    a. sorting the batteries in order to avoid inconsistency of the batteries to be connected in series;            b. charging the batteries separately (e.g. U.S. Pat. No. 6,586,909), in order to overcome the problems mentioned above, however, low voltage is required for charging each battery to full (for example, the lithium iron battery is charged to 3.65V) and this low voltage charging is not energy efficient owing to conversions from normal high voltage AC power source to low voltage DC power.Most prior art systems and methods utilized in making the batteries balanced during charging use complicated circuitry to detect and balance the uncharged batteries (e.g. U.S. Pat. No. 7,068,011, U.S. Pat. No. 7,061,207, U.S. Pat. No. 6,882,129, U.S. Pat. No. 6,841,971, U.S. Pat. No. 6,825,638, U.S. Pat. No. 6,801,014, U.S. Pat. No. 6,784,638, U.S. Pat. No. 6,777,908, U.S. Pat. No. 6,700,350, U.S. Pat. No. 6,642,693, U.S. Pat. No. 6,586,909, U.S. Pat. No. 6,511,764, U.S. Pat. No. 6,271,645).                        
Rechargeable batteries are becoming more and more important nowadays for wide range of applications other than conventional consumer electronics such as cell phone or lap top computers owing to their remarkable capability in power, enhanced cycle life, and the environmental benign nature. The major reason responsible for this change is that rechargeable batteries are becoming more reliable with better cycle life that allows people to start constructing bigger battery systems for heavier duty applications such as home energy saving systems, or even electric vehicle applications. However, with the increase in size meaning more battery cells being connected in series and parallel, the service life of the resultant battery system becomes unpredictable or even unreliable. One example is the battery cells constructed using the previously patented lithium iron phosphorous oxide (LiFe(1-x)MxP(1-x)O2(2-x)) material as the cathode. Although the single battery performs 1000 cycles easily with more than 80% capacity remaining, the increase in size (with more battery cells connected in series and parallel) could exhibit service life of the whole battery system ranging from several tens of cycles to thousands of cycles. The key to the cycle life enhancement is the capacity balance of battery cells which can be implemented during every time charging. From this perspective, the balance of battery cells is very much dependent on collaborative control being implemented on the battery cells and the charging method being utilized. In the present invention, a battery system is introduced by describing proper constraints and controls on battery cells and the charger or charging system that ensures the best implementation of the battery cells balance that leads to a prolonged cycle life of the whole battery system.